Recently, there is an accelerating tendency that mobile devices, such as mobile phones, personal digital assistances (PDA), and portable navigation devices, are thinned and downsized. The need for thinner and smaller components to be mounted in audiovisual equipment and the like has also been increasing.
In general, electrodynamic loudspeakers are used as a loudspeaker for reproducing audio signals or music signals in the mobile devices. The electrodynamic loudspeakers, however, employ a driving type that requires a magnet and a voice coil, and thus it is difficult to reduce the thickness of the loudspeakers. Further, the electrodynamic loudspeakers use a magnetic circuit, and thus a problem arises that countermeasures must be taken against magnetic leakage, or the like. Therefore, piezoelectric loudspeakers, which have been widely used for reproducing sounds in the audiovisual equipment or the like, are attracting attention as a loudspeaker of a driving type that is suitable for reducing the thickness. Thus, there is an increasing tendency to mount piezoelectric loudspeakers in mobile devices.
Conventionally, the piezoelectric loudspeaker has been known as an acoustic transducer, in which a piezoelectric member is used for an electro acoustic transducer element, and which is used as a sound output means of small-sized devices (e.g., see Patent Literature 1). A structure of the piezoelectric loudspeaker is such that the piezoelectric element is bonded on a metal plate, or the like. Therefore, the piezoelectric loudspeaker is readily reduced in its thickness, as compared to the electrodynamic loudspeaker that requires a magnet and a voice coil. The piezoelectric loudspeakers also have an advantage that no countermeasure is required against the magnetic leakage. When viewed as an electric element, the piezoelectric loudspeakers operate as a capacitor, while the electrodynamic loudspeakers operate mainly as a resistance component. Therefore, the lower the frequency is, the higher the electric impedance becomes, and thereby the piezoelectric loudspeakers have an advantage that the power consumption in a low-frequency band is significantly low, as compared to the electrodynamic loudspeakers. For example, when used in mobile devices, the piezoelectric loudspeakers can reduce the power consumption over the electrodynamic loudspeakers in a normal voice-band, particularly in a frequency band ranging from 1 kHz to 2 kHz.
On the other hand, the piezoelectric loudspeakers have a disadvantage that an amount of displacement of a piezoelectric diaphragm is small, as compared to the electrodynamic loudspeakers, when the same voltage is applied. Because of this, in a low-frequency band where a large displacement is required, a sound pressure becomes small (i.e., voltage sensitivity becomes low), and thereby a problem arises that audio signals cannot be reproduced with a sufficient sound pressure. Therefore, in order to overcome the above problems, one of the following methods needs to be chosen.
A first method is a method of enlarging the area of the piezoelectric diaphragm to obtain the sound pressure. If the amount of displacement of the piezoelectric diaphragm is constant, the sound pressure of the piezoelectric loudspeaker is proportional to an effective vibration area of the piezoelectric diaphragm, and therefore the effective vibration area is to be enlarged. For example, if the effective vibration area of the piezoelectric diaphragm is doubled, the sound pressure is also doubled, that is, a sound pressure level increases by 6 dB.
A second method is a method of increasing a driving voltage to obtain the sound pressure. If the effective vibration area is constant, the amount of displacement of the piezoelectric diaphragm of the piezoelectric loudspeaker is proportional to the driving voltage, and therefore the driving voltage is to be increased. For example, if the driving voltage is doubled, the sound pressure is also doubled.
A third method is a method of multi-layering the piezoelectric element to obtain the sound pressure. The number of laminations of the piezoelectric elements is to be increased because driving force of the piezoelectric loudspeaker is proportional to the number of laminations of the piezoelectric elements, if the total thickness of the piezoelectric elements and the driving voltage are constant in a state where directions of deformations of the piezoelectric members align with each other. Therefore, if the number of laminations of the piezoelectric elements is increased, the sound pressure of the loudspeaker increases without the need for changing the effective vibration area of the piezoelectric diaphragm and the driving voltage.